Hospital beds include a base frame, an elevatable frame including a deck, and a mattress supported by the deck. A typical bed also includes four siderail assemblies: left and right side assemblies near the head end of the bed, and left and right side assemblies near the foot end of the bed. Each siderail assembly includes a rail portion connected to the elevatable frame by links so that the rail, the elevatable frame and the links constitute a mechanism. One commonly used arrangement is a four bar linkage “drop down” mechanism in which the rail (one bar) is connected to the elevatable frame (the second bar) by two links (the third and fourth bars) so that the rail is vertically adjustable relative to the deck between a raised or deployed position and a lowered or stowed position. When the rail is in the deployed or stowed positions it assumes a substantially upright orientation in close lateral proximity to the sides of the mattress. At intermediate positions the rail remains substantially upright but is laterally displaced from the mattress by a rail offset distance. The offset distance depends on the mechanical arrangement of the mechanism links and joints and varies as a function of rail vertical position.
When the rail is in its raised position the top of the rail must project vertically higher than the top of the mattress by a minimum amount, e.g. 9 inches (approximately 22.9 cm). In addition, the bottom of the rail must be no more than a specified distance, e.g. 2 inches (approximately 5.1 cm) higher than the top of the deck. These requirements govern the minimum vertical dimension of the rail.
When the rail is in its lowered or stowed position the top of the rail should be no higher than a slight distance above the height of the lateral extremities of the deck in order to facilitate occupant ingress and egress. In addition, the bottom of the rail must be at least a minimum distance above the floor when the elevatable frame, and therefore the deck, is positioned at its lowest elevation. This minimum distance provides clearance for a caregiver to position the wheels of a rolling table under the bed. The clearance also guards against accidental entrapment of an obstruction located under the rail when the rail is in the stowed position and the elevatable frame is being lowered toward the floor. Providing this floor clearance is especially desirable if the rail is designed so that the rail is unable to move vertically upwardly relative to the elevatable frame upon contact with an obstruction. If the rail is able to move vertically upwardly relative to the frame upon contact with an obstruction, a smaller floor clearance may be acceptable, particularly if the rail offset distance is small.
It is desirable to be able to position the elevatable frame so that the deck is as close to the floor as possible. However the above described constraints on the rail vertical dimension and floor clearance act together to limit the minimum height to which the frame and deck can be lowered. Accordingly, designers seek ways to achieve the lowest possible minimum deck height while adhering to the constraints. As noted above, reducing the rail offset distance offers a possible way to trade floor clearance in return for otherwise unattainable reductions in minimum deck height.
Another desirable attribute of a siderail assembly relates to the orientation of the rail portion of the assembly. A conventional “drop down” siderail assembly remains in a substantially upright orientation irrespective of its elevation (deployed, stowed or somewhere in between). When an occupant wishes to leave the bed a caregiver lowers at least one of the rails and the occupant sits near the edge of the bed with the lowered rail behind her calves and with her feet on the floor. The presence of the rail causes the occupant's feet to be further away from the bed, and therefore further away from her seated center of gravity, than would be the case if the rail were absent. If, however, the rail were in an inclined, bottom-in/top-out orientation (i.e. with the bottom closer to the mattress) the occupant could position her feet closer to her seated center of gravity, thereby achieving better stability when transitioning from being seated on the mattress to standing on the floor or vice versa.
Some bed decks have a “step” architecture featuring a horizontal platform, a wall rising from the lateral extremities of the platform and a horizontal ledge extending laterally outwardly from the upper end of each wall. The ledges and walls account for about 20% of the lateral dimension of the deck (measured horizontally); the platform accounts for about 80%. The corresponding mattress includes a vertically thick center section and a pair of vertically thinner, laterally extending wings. The mattress center section rests atop the deck platform; the wings rest atop the ledges. One advantage of the step architecture is that the space outboard of the deck walls and beneath the ledges (i.e. outboard of the mattress center section and beneath the mattress wings) is available for occupancy by other bed components, such as the links that connect the rail to the frame. The availability of this space offers the mechanism designer flexibility and options in link and joint positioning and trajectory, thereby making it easier to design a linkage capable of satisfying potentially conflicting requirements.
The previously noted design requirements for the rail, including the need to provide ground clearance, can make it difficult to design a siderail assembly capable of positioning the rail in compliance with the positioning requirements and capable of accurately reproducing a desired rail trajectory. As noted, some relief from the floor clearance requirements may be obtained by reducing the rail offset distance, but this merely adds an additional, potentially complicating requirement to the rail trajectory. The design task is further complicated if it desired to achieve the above described bottom-in/top-out orientation of the rail in its stowed position and/or if the siderail assembly must be designed for use with a flat deck rather than for use with the step deck and its attendant advantages for positioning the links and joints. It is, therefore, desirable to devise a simple, cost effective mechanism capable of meeting the various requirements, including the bottom-in/top-out stowed orientation if desired, even if confined by the need to apply the siderail assembly to a bed having a flat deck.